I. Field of the Invention
The present invention relates to an electron gun used in an in-line type color CRT for displaying an image by converging a plurality of electron beams on a phosphor screen and to a color CRT using the same.
II. Description of the Related Art
An in-line type color CRT has usually three electron guns and is designed to display an image by converging three electron beams emitted from these electron guns onto a phosphor screen. FIG. 1 is a schematic sectional view of an in-line type color CRT. Phosphor screen 22 composed of three color phosphors is arranged on an inner surface of panel 21, which constitutes a front surface portion of envelope 20. Shadow mask 23 is arranged in envelope 20 at a predetermined distance from phosphor screen 22. Electron gun 26 is arranged in a neck 24 constituting a rear end portion of envelope 20. Three electron beams 25B, 25G, and 25R emitted from electron gun 26 pass through shadow mask 23 and are incident on phosphor screen 22, thereby displaying a color image.
Electron gun 26 comprises three cathodes 28a, 28b, and 28c, heaters (not shown) for independently heating these cathodes, and first to sixth grids 31 to 36 arranged in this order from the cathodes toward phosphor screen 22 along the axis of the CRT. Each grid is formed into a flat plane or a cylindrical shape having three holes allowing the electron beams to pass through them.
If the three electron beams are not properly converged on the phosphor screen, an incomplete display of an image is caused.
Convergence of the electron beams in a central area of the screen (static convergence) is adjusted by causing three unit electron guns to be inclined with each other, or tilting a main lens with respect to a passing direction of an electron beam. Convergence of the electron beams in a peripheral area of the screen (dynamic convergence) is adjusted by utilizing a convergence correcting unit, or self-convergence using nonuniform magnetic fields in a deflection yoke unit.
In the color CRT, a stable electron beam current can only be obtained by suppressing the so-called flying phenomenon, in which an electron beam current is greatly changed right after the heaters are lighted as compared with a stabilized current value. Especially in a type such as a display tube which displays a negative image such as a character or a figure on a non-emissive dark portion, degradation of the image contrast is caused by the flying phenomenon. In the color CRT, incomplete fidelity of color is also caused.
Japanese Utility Model Publication No. 60-35163 discloses a picture tube, wherein a first grid of an electron gun is composed of a member having a low thermal expansion coefficient (about 12.0.times.10.sup.-6) so as to decrease an amount of flying of an electron beam current and quickly stabilize the current. However, if the first grid is composed of a member having a low thermal expansion coefficient, static convergence is degraded. The reason will be described by exemplifying a unibipotential type electron guns generally used in a color CRT. As shown in FIG. 1, this electron gun comprises three cathodes 28a, 28b, and 28c, heaters (not shown) for independently heating these cathodes, and first to sixth grids 31 to 36 arranged in this order from the cathodes toward a phosphor screen. The cathodes, and the first and second grids constitute a triode. Voltages shown in Table 1 are normally applied to each cathode, each grid, and each heater. An electron beam emitted from each cathode which has received a video signal is cut off by the cathode voltage shown in Table 1.
TABLE 1 ______________________________________ Cathode 100 to 150 V First grid 0 V Second grid 300 to 1,000 V Third grid 16 to 35% of voltage applied to six grid Fourth grid Equal to voltage applied to second grid Fifth grid Equal to voltage applied to third grid Sixth grid 10 to 30 kV Heater 3 to 6.3 V ______________________________________
The first and second grids are control grids for accurately emitting electron beams in accordance with the video signal. The electron beams forms a crossover point once near the first or second grids, and then diverged into the third grid while being diverged. Then, the electron beams are focused by a main electron lens system constituted by the third to sixth grids, and formed images on the phosphor screen.
Accurate image formation can be realized only after the cathodes and respective grids have been heated by the heaters to the point where they are thermally stabilized.
Stable (maximum) temperatures for the cathodes and the respective grids are shown in Table 2. The periods of time required for raising the temperatures up to the respective stable temperatures are: about 5 seconds for the cathodes, about 10 minutes for the first and second grids, and about 15 to 20 minutes for the third to six grids.
TABLE 2 __________________________________________________________________________ Cathode 700 to 900.degree. C. First grid 150 to 300.degree. C. Second grid 100 to 200.degree. C. Third grid 80 to 120.degree. C. Fourth grid 50 to 100.degree. C. Fifth and six grids About 50.degree. C. __________________________________________________________________________
Each electrode is elongated in both an axial direction of the tube and a direction perpendicular to the tube axis until a corresponding stable temperature is attained.
The elongation along the tube axis causes the deviation of intervals between respective grids from the predetermined values, resulting in the flying phenomenon. Especially the change of interval between each cathode and the first grid has much influence upon the flying phenomenon. On the other hand, the elongation perpendicular to the tube axis causes the difference of three unit gun's separation among the respective grids, resulting in the degradation of static convergence. For this reason, the static convergence and cutoff are set after the electron gun is sufficiently heated.
If each grid is composed of a stainless member having an thermal expansion coefficient of about 17.0.times.10.sup.-6 at 0.degree. to 300.degree. C., the cathodes, and the first and second grids are elongated in the axial direction of the tube, as shown in FIG. 2.
The amount of flying of the electron gun is greatly influenced by the elongation of the first grid. The reason is as follows.
Cutoff voltage E.sub.C can be given by the following formula. The electron beam current is increased in proportion to the value of cutoff voltage E.sub.C. EQU E.sub.C .varies..phi..sup.3.E.sub.C2 /a.f.t
where
.phi.: a diameter of a hole of the first grid
a: a distance from the first grid to the cathodes
f: a distance from the first grid to the second grid
t: a thickness of the first grid
E.sub.C2 : a voltage applied to the second grid
In the above formula, .phi., t, and E.sub.C2 can be regarded as being always constant, whereas a and f are changed from the start time of the heaters. Assume that a and f upon lighting of the heaters and after they are sufficiently heated are respectively set as a.sub.1 and f.sub.1, and a.sub.2 and f.sub.2. If the product of a and f is constant, i.e., a.sub.1.f.sub.1 =a.sub.2.f.sub.2, a substantially ideal flying characteristic can be obtained, as indicated by curve 5 in FIG. 3. When a.sub.1.f.sub.1 &gt;a.sub.2.f.sub.2, a characteristic represented by curve 6 is obtained, and when a.sub.1.f.sub.1 &lt;a.sub.2.f.sub.2, a characteristic represented by curve 7 is obtained. Accordingly, if the first grid is composed of a member having a low thermal coefficient, changes in a and f can be reduced, and hence the characteristic of the electron beam represented by curve 6 or 7 can be made close to that represented by curve 5. Note that curve 8 represents the predetermined current value of the electron beam.
However, even if the first grid is composed of a member having a low thermal expansion coefficient so as to improve the flying characteristic, the static convergence cannot be improved, until respective grids have reached their stable temperature. This is because the centers of the holes of the respective grids are shifted from each other, until respective grids have reached their stable temperature, because of variations in elongation of the respective grids in the direction perpendicular to the axial direction of the tube due to differences between times required for attaining the respective stable temperatures of the third to sixth grids constituting the main electron lens system, as described above, thereby adversely affecting the convergence of the three electron beams.
FIG. 4 shows a measurement result of static convergence when each grid of the electron gun is formed by a generally used stainless member. The axis of abscissa represents an elapsed time from the start of the heaters. Curve 39 represents changes in static convergence with the lapse of time caused by misalignment of the centers of the holes between the first and second grids. Similarly, curves 40, 41, and 42 respectively represent changes in static convergence with the lapse of time caused by misalignment of the centers of the holes between the second and third grids, between the third and fourth grids, and between the fourth and fifth grids. Changes in static convergence as a whole with the lapse of time are represented by curve 43 obtained by adding curves 39 to 42 to each other. Accordingly, deviations of the centers are very large immediately after the heaters are lighted.
If the first and second grids are made of members each having a low thermal expansion coefficient of 12.0.times.10.sup.-6 or less in order to obtain an optimal flying characteristic during the output of an image, an underconvergence component in the static convergence indicated by curve 39 in FIG. 4 is reduced. Namely, as is apparent from the changes in static convergence with the lapse of time indicated by curve 44a or 44b shown in FIG. 5, the static convergence immediately after the lighting of heaters is excellent, however, overconvergence is increased with the lapse of time, and the peak value is attained after three minutes. As a whole, the static convergence characteristic is worse than that in the case using the stainless member. Curve 44a represents changes in static convergence when the first and second grids are made of a 42% Ni-Fe alloy (NSD) having a thermal expansion coefficient of 5.0.times.10.sup.-6 at 0.degree. to 300.degree. C., and the third grid et seq. are made of stainless steel having a thermal expansion coefficient of 17.0.times.10.sup.-6. Curve 44b represents changes in static convergence when the first grid is made of a 50% Ni-Fe alloy (TNF) having a thermal expansion coefficient of 9.4.times.10.sup.-6 to 10.4.times.10.sup.-6 at 30.degree. to 400.degree. C., and the second and third grids et seq. are respectively made of NSD and stainless steel. Assume that the thermal expansion coefficients of the first, second and third grids et seq. are respectively set to be .alpha..sub.1, .alpha..sub.2, and .alpha..sub.3. Then, in both curves 44a and 44b, EQU .alpha..sub.2 .ltoreq..alpha..sub.1 &lt;.alpha..sub.3
As is apparent from the comparison between curves 44a and 44b, in order to further reduce changes in static convergence with the lapse of time, thermal expansion coefficient .alpha..sub.2 of the second grid 2 may be further decreased. However, no member is found, which can satisfy characteristics required for grids, and has a thermal expansion coefficient smaller than NSD.